This invention relates to a thermostatic control especially adapted to a heat/cool room air conditioning system and more specifically to a control which provides variable temperature differential control for systems of this type in which the air temperature sensor is located at a position outside the room's normal comfort zone.
Zonal type room air conditioning systems that both heat and cool the room space are commercially available and are widely employed where individual room control is desired. Motels and office buildings are representative examples of such use. Typically, units of this type are mounted near the floor, for example, they may be mounted under a window in a through-the-wall configuration. It is customary to package and install these systems as a single complete unit. In some cases, the units employ a vapor compression refrigerant system for cooling purposes with electrical resistance heaters for heating purposes. In other cases, the units employ a heat pump system for both heating and cooling with supplemental resistance heaters included to assist in the heating operation on particularly cold days. These units generally also embody a thermostatic control with an air temperature sensor included in the installed unit. Although installation near the floor is a common practice, other installations may also be found such as over the door or window near the ceiling.
A disadvantage with either of these low or high installation configurations is that the temperature sensor when included within the unit is thus positioned outside the normal comfort zone of the room. That is to say, the occupants of the room most often physically sense, and are therefore made comfortable or uncomfortable by, temperature conditions that exist in approximately the middle one third of the room height. However, with either of the above-described unit installations, the temperature sensor is required to respond to temperature conditions existing outside this zone, either below or above the zone depending on the location of the air conditioning system.
With a single mode air conditioning system, for example, one that only cools the air space, the thermostatic control can be adjusted to compensate for this asymmetric location of the sensor such that desired temperatures are achieved in the comfort zone based on an assumed average temperature difference that is experienced between the comfort zone and the physical location of the temperature sensor. Such an assumption does not work in the case of a dual mode heat/cool system, however, since air temperature stratification in the room has an opposite influence on the disparity between the sensed air temperature and the actual comfort zone temperature as between the heat and cool operating modes. Thus with the temperature sensor located near the floor, the temperature sensor is influenced more readily in the cool operating mode by the heavier cool air coming from the air conditioner than it is in the heat mode by the lighter warm air. This is because the lighter warm air rises and takes longer for the accumulated warm air to reach the sensor than in the case of the heavier cool air which tends to accumulate near the floor first. This, of course, is a well-known phenomenon which can be seen to work in reverse in the case of an air conditioner mounted near the ceiling.
In a heat/cool air conditioning system employing a vapor compression refrigerant system, it is common to provide temperature control by cycling the compressor on and off, as necessary, to transfer heat into or out of the room in sufficient amounts to achieve the desired temperature level in the room. Heretofore, electromechanical sensors and control devices have been employed to achieve the appropriate cycling function. In order that the compressor not be damaged by an excessive cycling rate these controls have been provided, for example, with internal heating devices to provide a differential in the sensed temperature at which the compressor is cycled on and off. Thus a temperature range or differential is permitted in the room with the control being adjusted to compromise between, on the one hand, the amount of differential permitted without undue discomfort to the occupants and, on the other hand, the time delay between compressor on and off times required to avoid damage to the compressor or other adverse effects on its long term performance.
While devices of this type have been generally satisfactory, the advent of electronic controls makes it desirable to provide an electronic control of simple and inexpensive design that offers the functional control of compressor cycling in a heat/cool air conditioning system with the provision for the required temperature differential between on and off compressor cycle control. In particular, it is desired to provide such a control for a heat/cool room air conditioning unit having a temperature sensor located outside the room's comfort zone that varies this control temperature differential to compensate for the opposite influence that air stratification has in heat and cool operating modes on the temperature difference between the comfort zone and the location of the air temperature sensor.